Recently, a high-power fiber laser that uses a rare-earth doped optical fiber has been attracting attention. In a high-power fiber laser, as pump light and signal light propagate along a fiber, excited rare-earth ions amplify the signal light; the laser is also advantageous in that it is easy to cool during operation and can be made smaller.
An amplification fiber for use in such a high-power fiber laser has a double-clad structure in order to launch high-power pump light. FIG. 1 is a cross-sectional view of an example of a fiber having a double-clad structure. The double-clad fiber 1 in FIG. 1 has a two-layer cladding structure that includes a first cladding 12 and a second cladding 13 arranged around a core 11. The first cladding 12 acts as a cladding for the core where signal light is guided to, and the second cladding 13 acts as a cladding when the pump light is guided in multi-mode through the entire first cladding 12. Since the first cladding 12 normally has a diameter equal to or more than 100 μm, the launch efficiency of the pump light can be increased.
Furthermore, for a high-power fiber laser, a fiber that has a wavelength filter effect is demanded to propagate signal light intended for transmission, while suppressing propagation of light at wavelengths other than the signal light, e.g. amplified spontaneous emission (ASE) and stimulated Raman scattering. As a fiber having such an effect, there is proposed a photonic bandgap fiber which only guides light in a predetermined wavelength by Bragg reflection by disposing a periodic structure around the core. As disclosed in Patent Documents 1 and 2 and Non-Patent Document 1, particularly useful is a solid-core photonic bandgap fiber having a core that is entirely made of a solid material; this fiber allows easy fusion splicing, and can be used as an amplification fiber by doping the core with a rare-earth element. FIG. 2 is a cross-sectional view of an example of a conventional solid-core photonic bandgap fiber, where reference numeral 2 represents a solid-core photonic bandgap fiber, 21 represents a core, 22 represents a cladding, and 23 represents high-refractive-index portions having a higher refractive index than the cladding. The solid-core photonic bandgap fiber 2 includes a great many high-refractive-index portions 23 which are arranged in a periodic structure around the region of the core 21, and guides light by Bragg reflection.    [Patent Document 1] Japanese Patent Publication No. 3072842    [Patent Document 2] WO02/101429 Pamphlet    [Non-Patent Document] ‘Design of microstructured single-mode fiber combining large mode area and high rare earth ion concentration’ Optics express, vol. 14, No. 7, pp. 2994, 2006.